This invention relates to an inverter system having a plurality of modules connected in parallel to a load, and more particularly to an arrangement that applies a composite feedback current derived from all of the modules to the base drive of each of the modules, thereby to provide the entire system with protection against open or short circuit faults occuring in any of the modules.
The type of circuit selected for driving the output power transistors of paralleled inverter modules has a large influence on system efficiency, redundancy capability, load sharing and overload tolerance. In the past, there have been two base drive circuits customarily used for the power output transistors of paralleled modules of the same push-pull type employed in the present invention, but in one the base drive circuit is without any current feedback from the load, and in the other shown in FIG. 1 the base drive circuit is with output current feedback from each individual module. Thus, in one prior-art type of circuit customarily used for driving the output transistors of a multi-module, paralleled inverter system, no current feedback is provided for control, and in the other type of circuit customarily used, feedback in each module is limited to control of its own drive.
Both prior-art arrangements have deficiencies. In the case of no feedback, the base current of push-pull power transistors is held constant regardless of the output load value. Therefore power dissipation in the base circuit is the same regardless of the load so that efficiency decreases with lighter loads. In the case of individual output current feedback, base current of the power transistors is substantially proportional to the output load demand, and power dissipation is thus nearly proportional to the load to maintain efficiency reasonably high and constant despite load variation.
The first type of circuit has another disadvantage on comparison to that of the second type under some overload conditions, such as when any one of the paralleled outputs of the entire inverter system short circuits. In that event, the affected inverter module must be able to sustain the temporary overload until a protective fuse in series with the load opens, but since there is no feedback arrangement to control the base drive, the power transistor may not saturate and this can result in transistor failure because of excessive power dissipation in the collector-emitter regions. The circuit could be designed to provide sufficient base current capacity for driving the overload, but if that is done, total system efficiency is decreased.
If, on the other hand, a feedback arrangement is provided to control the base drive with individual output current feedback, as shown in FIG. 1 the result is unequal current sharing among power transistors, particularly during the power transistor turn-on period since each then has its own feedback control. Assuming an arrangement in which N modules are used, each with a pair of transistors Q.sub.A and Q.sub.B, the power transistors Q.sub.B1 . . . Q.sub.BN are then turned on at the same time during the alternate half cycle of a control clock signal. Now if the transistor Q.sub.B1 starts to turn on faster than another corresponding transistor, such as transistor Q.sub.BN, the output current of transistor Q.sub.B1 is larger than that of transistor Q.sub.BN because of transistor response difference. Therefore, transistor Q.sub.B1 is accelerated to turn on further than during the slow turn on of transistor Q.sub.BN. This results in high spike generation during turn-on of transistor Q.sub.B1. Such an effect might stress the faster switching transistor sufficiently to produce failure of that transistor. Moreover, such an individual output current feedback arrangement can cause undesirable inverter system oscillation in the event of an open circuit in one of the two primary windings of the push-pull operated transformers T.sub.1 employed to control base current in the power transistors.
An object of this invention is to provide an arrangement for applying a composite feedback current derived from all of the modules of paralleled inverter modules to the base drive of each of the modules thereby to provide the entire system with protection against open or short circuit faults occuring in any of the modules.